A conventional bicycle rim is made of metal such as Aluminum alloy which is strong and easily formed. Nevertheless, the conventional bicycle rim is heavy and cannot satisfy the needs from the market, especially for racing bicycles and road bicycles. In order to reduce the weight of the bicycle rims, some manufacturers use fibers material o be the inner portion of the rim which includes an outer portion made of metal. This improvement can only achieve limited goals for having a light bicycle rim.
A further improvement for the bicycle rim is to make the whole bicycle rim by fibers material. The method for making the light bicycle rim uses a bladder completely or partially enclosed by the fibers material and both of which are then arranged into a mold set. The bladder is inflated and contacts against the fibers material to form the chamber in the bicycle rim and the fibers material is heated under a pre-set pressure, so that when the fibers material becomes stiff in the mold, the bicycle rim is formed with the light material and a space is defined within the rim. However, the thickness of the bladder cannot be controlled perfectly so that the thickness of the rim varies. Therefore, the manufacturers have to proceed extra trimming or machining steps to the rim so as to meet the expected design purposes. If the shape of the annular curved groove becomes a U-shaped groove with straight sidewalls then the structural strength of the bicycle rim will be weakened.
The present invention intends to provide a bicycle rim made of composite material and the method for making the rim includes two times of heat-molding to respectively make the outer part and the inner part of the rim so as to obtain the strong and light bicycle rims.